Season the thighs with the fajita seasoning. Don't be shy with the seasoning. Let the chicken set for 15 minutes or up to 2 hours.
Preheat a griddle or large skillet over high heat.
Add 2 tablespoons of butter to the skillet. Add the thighs and cook for 4 minutes per side until lightly charred and cooked to 165 F.
Remove the chicken to a platter to cool slightly then cut into small cubes.
Add remaining butter to the skillet and add the onions and peppers. Cook until lightly softened and charred, stirring often.
Add the chicken to the vegetables. Season with more fajita seasoning and stir to combine.
Add half of the cheese and mix until melted.
Reduce heat to low and cover until ready to use.
For the potatoes
Preheat oven to 400 F.
Rinse the potatoes under cold water and brush off any dirt. Dry well with a paper towel.
Pierce each potato 4 times with a sharp knife.
Brush potatoes with oil.
Sprinkle some salt onto a plate and roll the potatoes in the salt.
Place potatoes directly on the oven center rack. You might want to put a baking sheet lined with foil beneath them to catch any drippings.
Bake until potatoes are done (210 F internal temperature, or tender all the way through when poked with a knife), 40-60 minutes depending on their size. You can also test for doneness by squeezing the potatoes (with potholders on). If they give easily, they are done.
Remove the potatoes from the oven and let cool slightly before slicing.
Season insides with salt and pepper (or use more fajita seasoning and then divide the chicken mixture between the potatoes. Pile it good and high.
Top with remaining cheese and place in the oven for 5 minutes to melt.
Serve topped with crema and chopped cilantro.
Notes
You can substitute shredded Monterey Jack or pepper jack for the cheese.